1. Field of Invention:
This invention pertains generally to the field of stabilized zirconia, and more specifically to the field of thermally stabilized zirconia.
2. Description of Prior Art:
Thermal barrier coatings provide thermal insulation to gas turbine engine hot-end components, such as blades and vanes. The application of ceramic coatings for thermal insulation significantly increases the efficiency and the power of the engine by lowering the temperature of the metal components while reducing engine fabrication costs by avoiding complex cooling schemes. Zirconia is the ceramic material that has been studied most extensively for thermal barrier coating applications. The properties of zirccnia that are attractive for thermal barrier coatings include high melting temperature, low thermal conductivity, and a relatively high thermal coefficient of expansion to match expansion of the underlying superalloy substrate, if one is present. However, pure zirconia has a very serious drawback related to its high temperature phase stability. Depending on the temperature, zirconia has three distinct crystal structures. The cubic phase is stable at temperatures above about 2370.degree. C. A tetragonal phase is stable between about 1150.degree. C. and 2370.degree. C. Below about 1150.degree. C., the tetragonal phase transforms to a monoclinic phase through a martensitic transformation. This transformation is accompanied by a 3-6% volume expansion and the generation of shear strains due to distortion of the tetragonal crystal. The tetragonal-to-monoclinic transformation is troublesome for coating applications since it leads to coating disintegration and failure after only a few operating cycles.
Different approaches have been tried to stabilize the tetragonal phase zirconia. The most common and accepted method for phase stabilization is alloying zirconia (ZrO.sub.2) with either yttria (Y.sub.2 O.sub.3), ceria (CeO.sub.2) or magnesia (MgO). At present time, yttria-stabilized zirconia is being evaluated for thermal barrier coating applications in gas turbine engine airfoils and other high temperature gas engines. The zirconia-based thermal barrier coating system used in turbine engine components is of a multilayer construction, with an MCrAlY (M=Ni, Co, or Fe) inner coating, known as the bond coat, on a superalloy substrate and yttria-stabilized zirconia outer coating. An alumina (Al.sub.2 O.sub.3) scale forms at the interface between the MCrAlY and the yttria-stabilized zirconia layers during normal engine operation.
Failure of the thermal barrier system, often after only a few operating cycles, is often caused by cracks that develop between either the alumina scale and the yttria-stabilized zirconia coating or the scale and the bond coat. In addition to this mechanical failure mechanism, yttria is known to be subject to severe hot corrosion in the presence of salt, such as in a marine environment, and by sulfur and vanadium, which are common fuel contaminants. This hot corrosion process, leading to leaching of the yttria out of the yttria stabilized zirconia layer, effectively destroys the phase stabilization action of yttria.
U.S. Pat. No. 5,147,731 discloses a corrosion resistant structure of an outer ceramic layer disposed over a base alloy of MCrAlY when the outer ceramic layer is alumina stabilized zirconia consisting of 40-50 weight percent zirconium, 32-36% oxygen, and 18-24% aluminum.